Integral boiling and superheating neutronic reactor



Feb. 27, 1968 INTEGRAL BOILING AND SUPERHEATING NEUTRONIC REACTOROriginal Filed April 28, 1961 9 Sheets-Sheet 1 Feb. 27, 1968 s. N. TOWERET AL 3,371,016

INTEGRAL BOILING AND SUPERHEATING NEUTRONIC REACTOR Original Filed April28, 1961 9 Sheets-Sheet 2 (D ID Feb. 27, 1968 TOWER ET AL 3,371,016

INTEGRAL BOILING AND SUPERHEATING NEUTRONIC REACTOR 9 Sheets-Shem, 3

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INTEGRAL BOILING AND SUPERHEATING NEUTRONIC REACTOR Original Filed April28, 1961 9 Sheets-Sheet 4 Feb. 27, 1968 S. N. TOWER ET AL INTEGRALBOILING AND SUPERHEATING NEUTHONIC REACTOR Original Filed April 28, 19619 Sheets-Sheet E) INVENTORS Stephen N. Tower and William H. Arnold, Jr.

ATTORNEY Feb. 27, 1968 s. N. TOWER ET AL 3,371,016

INTEGRAL BOILING AND SUPERHEATING NEUTRONIC REACTOR 9 Sheets-Sheet 6Original Filed April 28, 1961 Feb. 27, 1968 s. N. TOWER ET AL 3,371,015

INTEGRAL BOILING AND SUPERHEATING NEUTRONIC REACTOR Original Filed April28, 1961 9 Sheets-Shea Fig.6.

Feb. 27, 1968 s. N. TOWER ET AL 3,371,016

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INTEGRAL BOILING AND SUPERHEATING NEUTRONIC REACTOR Original Filed April28, 1961 9 Sheets-Sheet 9 mmm X3 mozmomj United States 3,3 7 1,6 lPatented Feb. 27, 1 $68 3,371,016 INTEGRAL BGELING AND SLWERHEATINGNEUTRONIC REACTOR Stephen N. Tower, Murrysville, and Wiiiiam H. Arnold,

Sin, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation,Pittsburgh, Pa., a corporation of Pennsylvania Original application Apr.28, 1961, Ser. No. 166,247, new Patent No. 3,249,506, dated May 3, 1966.Divided and this application Mar. 1, 1966, Ser. No. 530,930

3 Claims. (Cl. 176-53) This invention is a division of copendingapplication Serial No. 106,247, filed April 28, 1961, now Patent No.3,249,506 entitled Integral Vapor Generating and Superheating NeutronicReactor System, and also assigned to the present assignee.

This invention relates in general to a direct cycle neutronic reactorand more particularly to an integral vapor generating and superheatingreactor, including in certain modifications thereof a vapor reheatingcycle.

As is well known, a neutronic reactor is arranged for transferring theheat developed in the fissioning process maintained in the core of thereactor to a suitable working fluid such as water, steam, or acombination thereof. Such fissioning is maintained by a chain reactionin a mass fissionable isotope, such as U U PLI239, or combinationsthereof, confined within the core of the reactor. The fissioning processis induced by the capture of a thermalized neutron which, in turn,results in the splitting of the fissionable atom into additionalneutrons and fission fragments. The latter neutrons are categorized asfast and are thermalized by moderator material admixed with orjuxtaposed to the fissionable material. The fissioning rocess,therefore, becomes chain reacting as long as sufficient thermalizedneutrons are made available for each succeeding generation of fissions.The fissioning and moderating materials usually are surrounded by aneutron-reflecting material for improvement in neutron economy. Thethermalized neutron flux level, however, is controlled by thepositioning or presence of control rods employed in the reactor.

Direct cycle neutronic reactors of the prior art pro duced saturatedvapor, such as saturated steam, which is used in external vaporutilizing means, such as steam turbines. However, standard steam,turbines of todays power industry utilize high pressure superheatedsteam. These standard steam turbines are more eflicient and costconsiderably less than a saturated steam turbine. Todays power industryalso utilizes a reheat cycle in a conventional power plant in order toobtain even higher efficiencies than can be obtained by utilizing only asuperheated steam cycle. In the conventional reheat cycle, the highpressure superheated steam is passed through the high pressure andintermediate pressure sections of the turbine. The steam is thenreturned to the conventional steam generator, reheated, and then sent toa lower pres sure section of the turbine.

Accordingly, it is the general object of this invention to provide anovel and more eificient direct cycle neutronic reactor.

It is a more particular object of this invention to provide a novel andmore efiicient integral boiling and superheating neutronic reactor,which is sometimes hereinafter referred to as IBSHR.

Another object of this invention is to utilize a pressure tube designthrough which a flow of high pressure primary coolant can be maintainedto transfer heat from the fissile fuel also contained within thepressure tube.

Still another object of this invention is to utilize a reentrant typepressure tube which sealably passes through only one end of the reactorvessel and thereby eliminates the differential expansion problem betweenthe re-entrant tube and the reactor vessel.

Still another object of this invention is to minimize the weight, spacerequirement, and power requirements for auxiliary equipment byeliminating vapor pumps, for example as used in the Loefller cycle,required to force saturated vapor through separate external vaporsuperheaters and eliminating heat sources external to the reactor usedfor superheating saturated vapor. 7

Another object of this invention is to use a moderator from which heatcan be recovered to heat or preheat a primary coolant, such as a liquid,saturated vapor, or superheated vapor, flowing through the reactor core.

Still another object of this invention is to contain the moderator in alow pressure vessel within which a fluid coolant arrangement such as aninert gas blanket is maintained and circulated to facilitate heattransfer from the moderator, which operates at a high temperature, tothe incoming primary coolant being circulated through the pressuretubes.

Still another object of this invention is to provide a novel and moreeflicient IBSHR by reheating within the same reactor a vapor, which hasalready been utilized by a vapor utilizing means.

Briefly, the present invention accomplishes the above cited objects byproviding an integral boiling and superheating reactor (IBSHR)comprising a separate moderator structure or lattice which in theexamples herein utilizes a solid moderator material, a pressure tubedesign, and a two region reactor core in certain modifications thereofor a three region core in others. The moderator, for example, graphite,is contained in a low pressure tank. The pressure tubes, which containboth fissile fuel and primary coolant during reactor operation, arepositioned vertically as viewed in the drawings and are uniformly spacedwithin the graphite moderator. The reactor core is divided into aplurality of regions with two or three being utilized in the examplesherein depending on the use of reheat. Primary coolant, for example,primary water, enters the center region or boiling pressure tubes,absorbs heat from the fissile material contained inside the boilingpressure tubes, and is converted into a steam-water mixture. Thesteam-water mixture thus produced is then sent to a vapor separator, forexample, a steam drum, where the steam is separated from the water. Theseparated steam, which at this stage is saturated steam, is then sent tothe pressure tubes or reactor superheat tubes in another region of thecore. The saturated steam then flows through the reactor superheat tubesand absorbs heat from the fissile fuel contained therein to becomesuperheated steam. The superheated steam is then sent directly to theturbine to generate electric power. In the aforementioned example of atwo region core it is also possible to reverse the locations of theboiling and superheat regions of the reactor core, so that the boilingregion can be located on the periphery and the superheat region in thecenter of the reactor core.

In addition, a blanket of secondary fluid or inert gas, for examplehelium, desirably is maintained within the low pressure reactor vesselcontaining the graphite moderator and pressure tubes. Since the graphiteis operated in a hot condition (approximately 1200 to 1300 F.),substantially all of the moderator heat is transferred back to thepressure tubes where it is used to produce saturated and superheatedsteam. To facilitate the heat transfer from the graphite moderator tothe primary coolant passing through the pressure tubes, the inert gas iscirculated by a blower. In other applications of the invention, theheated helium can also be cooled in an external heat exchanger.

An additional reactor arrangement as taught by the invention is to add athird region to the reactor core. This steam to rise. The reheatedsteamis then sent from the reheat tubes to a lower pressure section ofthe steam turbine where additional electric power is generated.

These and other objects and advantages of the present invention willbecome more apparent when considered in view of the following detaileddescription and drawings, in which:

FIGURES 1a and 1b are a vertical section through the reactor shownschematically in FIG. as taken generally along the reference line I--Iof FIG. 2;

FIG. 2 is a partial cross-sectional view of the reactor shown in FIG. land taken along reference line IIII thereof;

FIG. 3 is an enlarged view of the upper right hand area of FIG. 1,showing details in that area with greater clarity;

FIG. 4 is a top plan view of the vapor separators and .conduitarrangement between the vapor separators and reactor, with across-sectional view of the reactor taken along the reference line IV-IVof FIG. 1 to show more clearly the baffle arrangement at thetop of thereactor;

FIG. 5 is a schematic fluid circuit diagram of the vapor generatingsystem with certain auxiliary equipment and external vaporutilizing-means;

FIG. 6 is a vertical sectional view of of the reactor shown in FIG. 1;

FIG. 7 -is a simplified vertical sectional view of a straight throughtype pressure tube adapted for use in the reactor system disclosedhereinafter;

FIG. 8 is a simplified vertical sectional view of a twopasstype pressuretube adapted for use in the reactor sysa modified form tem disclosedhereinafter;

Stayed-tube sheet type IBSHR Referring now to FIGS. 1 through 3 of thedrawings, an illustrative example of the direct cycle, integral boilingand superheat reactor (IBSHR) of the pressure tube type is depictedtherein. Also in FIG. 2 a single circle represents an upper boiling orsuperheat stay, and a double circle represents an upper control rodstay. In this embodiment, a reactor 16 of the stayed-tube type is shown.A reactor vessel 18, fabricated generally in the form of a rightcircular cylinder and having stayed-tube sheets at its ends, forms theouter shell of the reactor 16. An upper stayed-tube sheet 20 and a lowerstayed-tube sheet 22 each having a spherically shaped outer plate 24 anda fiat inner plate 26 forming an enclosed upper plenum chamber 28 and anenclosed lower plenum chamber 30. At the periphery of the lowerstayed-tube sheet 22, is formed a liquid or water inlet nozzle 32, whichcouples the lower plenum chamber 30 to a circulating pump dischargeconduit 34. A plurality of lower stays 36, 38 and 40 are verticallydisposed within the lower plenum chamber 30 and pass through openings inthe fiat inner plate 26 and the spherically shaped outer plate 24 of thelower stayed-tube sheet 22. The lower stays 36, 38 and 40 are sealablystrength-welded to the spherically shaped outer plate 24 and the flatinner plate 26 to prevent the plates 24 and 26 from blowing apart 4 as aresult of the internal pressure within the lower plenum chamber 30.

The lower control rod stays 36 extend downwardly to a control rod drivemechanism (not shown) and are hermetically sealed thereto. A control roddrive shaft 42 connects the control rod drive mechanism (not shown) to acontrol rod 44 and passes through the lower control rod stay 36. Thecontrol rods 44 are distributed substantially uniformly throughout anactive core 46 to be described more fully hereinafter. This large numberof control rods 44 is not'needed to shut down the reactor 16, but isneeded to assure proper power distribution during the operational lifeof the active core 46. The control rods 44 are cylindrical in shape andconsist of boron carbide (13 C) inserted in a thin wall stainless steelcan. The control rod drive shaft 42 is a thin wall stainless steel tubefilled with graphite. The control rods 44 are driven by conventionalrack and pinion control rod drives (not shown) located below andexternal to the reactor 16. No shaft seals will be used, as the drivemotors (not shown) will be canned to prevent the loss of the secondarycoolant or helium being circulated within the reactor vessel '18 forcooling purposes. The control rods 44 will also be vented directly intothe space enclosed by the reactor vessel 18 to prevent pressure buildupand subsequent distortion of the control rods 44 by helium gas producedby the absorption of neutrons in the boron poison.

As shown in thedrawings, the control rods 44 are inserted fromthe bottomof the reactor 16. In this arrangement of the invention, the controlrods 44 coextend with the entire height of the active core -16, thetotal height of which is represented by the length of fuel assemblies112. The uppermost position of the control rod 44 is that shown at 44'.However, the lowermost position of the control rod 44 is that shown at44" so that the control rods 44, when scrammed, dropto a centralposition within the active core'46. When thecontrol rods 44 arewithdrawn from the active core 46, they are displaced upwardly and thedriving mechanism therefor is arranged in a conventional manner torelease the control rods 44 upon failure of electric power. Accordingly,the control rod 44 arrangement is completely failsafe inasmuch as'noforce accumulating means such as springs or pneumatic 'tanks arerequired to force-the control rods into the active core 46 in the eventof power failure.

Each of the lower boiling stays 38 has a plurality of elongated holes48, which couple the'lower plenum chamber 30 to the inside of the stay38 to permit the passage of primary water from the plenum chamber 30 tothe inside of the lower boiling stay 38. An end plug 50 (FIG. 3) is usedwithinthe top portion of'the end plug 50 being sealably welded to thelower boiling stay 38. An alternative methodof plugging the lower openend of the lower boiling stay is described and claimed in a copendingapplication of P. J. Collins et al., entitled Nuclear Reactor RefuelingSystem, 'filed August 31, 1959, Serial No. 837,091, now US. Patent No.3,167,481, issued January 26, 1965, and assigned to CanadianWestinghouse Company Limited.

The lower superheat stays 40 are disposed within the lower plenumchamber 30 and secured to the lower stayed-tube sheet 22 in the samemanner as previously described for the lower boiling stays 38. Also thelower end of each lower superheat stay 40 protrudes a short distancebeyond the outer plate 24 of the 'lower stayedtube sheet 22. Througheach of the lower superheat stays 40 slidably passes a superheat tube54. The superheat tubes 54 couple reactor superheat tubes 60, to bedescribed hereinafter, to a ring shaped superheat outlet header 58 whichhas a tubular cross-section. To the lower end of each of the lowersuperheat stays 40 is sealably secured a bellows type expansion joint62, and the lower end of the bellows joint 62 is sealably secured to thesuperheat tubes 54. The bellows joint 62 is of conventional constructionand need not be elaborated upon here. The bellows joint 62 has a dualfunction in that it prevents the escape of the helium gas, containedwithin the reactor vessel 18, through the clearance between thesuperheat tube 54 and the lower superheat stay 40 and also provides ameans for taking care of the diiferential expansion which occurs betweenthe reactor superheat tubes 60 and reactor boiling tubes 64 due to atemperature dilierence existing between the reactor superheat tubes 60and the reactor boiling tubes 64.

Returning now to the upper stayed-tube sheet 20, there is formed at itsperiphery a plurality of steam-water outlet nozzles 70 and a pluralityof steam inlet nozzles 72. The nozzles 70 and 72 are alternativelyspaced as shown in FIG. 4. The plenum chamber 28, within the stayed-tubesheet 20, is divided into two sealably separated spaces, a steam-waterspace 66 and a steam space 68. The steamwater outlet nozzle 70 couplesthe steam-water space 66 to a steam-water conduit 74, which in turn iscoupled to the lower end of a conventional vapor separator or steam drum76 (FIG. 4). As shown in FIG. 4, two adjacent steam-water conduits 74enter the lower end of the steam drum 76. A steam drum outlet conduit 78(FIG. 4) then couples the top of the steam drum 76 (FIG. 4) to twoadjacent reactor steam inlet conduits 80 (FIG. 4). The steam inletnozzle 72 then couples the reactor steam inlet conduit 80 to the steamspace 68 within the upper plenum chamber 28.

FIG. 4 also shows that there are three steam drums 76 coupled to thereactor 16 and that there is a separate fluid circuit connecting eachsteam drum 76 to the reactor 16. In each circuit the steam-water mixturefrom b the steam-water space 66 flows to the steam drum 76, as indicatedby the dot-dash line flow arrow 222. The steam is separated from thesteam-water mixture in the steam drum 76. The steam then returns fromthe top of the steam drum 76 and enters the steam space 68 within theupper stayed-tube sheet 20, as indicated by the broken line flow arrow224.

Returning now to FIGS. 1 through 3, the steam space 68 within the plenumchamber 28 is formed by a vertical steam baffle 82, a horizontal steambathe 84, and a portion of the outer plate 24. The vertical steam batfie82 is sealably welded to the spherically shaped outer plate 24, extendsdownwardly to a point above the steam-Water outlet nozzle '70, and isjuxtaposed from the periphery of the upper stayed-tube sheet 20 in agenerally hexagonal shape as shown in FIG. 4 or as shown by referenceline 86 (FIG. 2). The horizontal steam bane 84 is sealably welded to thebottom of the vertical steam baffle 82 and also to the periphery of theupper stayed-tube sheet 20. Therefore, this permits the steam-watermixture in the steam-water space 66 to flow under the steam space 68 andthrough the steam-water outlet nozzle 70.

At each of the steam inlet nozzles 72, a space is enclosed by thehorizontal steam baffle 84, the fiat inner plate 26, and a five sidedsteam inlet baffle 88 as shown more particularly in FIGS. 2 and 4. Thesteam inlet baffle 88 is vertically positioned, encloses a spacesurrounding the steam inlet nozzle 72, and is sealably welded to boththe horizontal steam baflle 84 and the flat inner plate 26. At each ofthe steam inlet nozzles '72, the horizontal steam baflie 84 is cutout soas to form an opening equal to the area enclosed by the steam inletbaflie 88 in order to permit the steam entering through the steam inletnozzle 72 to flow into the steam space 68. The vertical steam baflle 82and the horizontal steam baffle 84 have been broken away at referencecharacter 0 (FIG. 4) in order to indicate more clearly the existence ofthe horizontal steam bafile 84. FIG. 4 shows a plan view of thehorizontal steam baflle 84 located between the vertical steam bafile 82and the outer periphery of the upper stayed-tube sheet along with thecutouts in the horizontal steam baffle 84 which conforms to the spaceenclosed by the steam inlet baffie 88.

A plurality of upper control rod stays 92, upper boilin stays 94 andupper superheat stays 96 are vertically disposed and secured within theupper plenum chamber 28 in the same manner as was previously describedfor the lower stays 36, 38 and 40 within the lower plenum chamber 30. Inaddition, the upper control rod stays 92 and the upper superheat stays96, which pass through the horizontal steam bafile 84, are sealablywelded to the horizontal steam batlle 84. All the stays 92, 94 and 96protrude a short distance above the spherically shaped outer plate 24and have end plugs 50 or the alternative method of plugging aspreviously described in connection with the lower boiling stays 38. Theupper stays 92, 4 and 96 are located directly above and aligned withtheir respective lower stays 36, 38 and 40. The upper control rod stays92 permit the control rods 44 to enter slidably therein. The upperboiling stays 94 have a plurality of upper boiling stay holes 98 locatedat the lower end of each boiling stay 94 to permit the flow ofsteam-water mixture from inside the upper boiling stays 94 to thesteam-water space 66. Each of the upper superheat stays 96 has aplurality of upper superheat stay holes 100 so as to permit the flow ofsteam from the steam space 68 into the upper superheat stays 96.

A superheat extension tube 102 is inserted a short distance into thelower portion of each upper superheat stay 96 and welded to the innersurface of the stay 96. The superheat extension tube 102 extendsdownwardly in a vertical position to a point approximately at the centerof an upper shield 104 to be described hereinafter, The verticallypositioned reactor superheat tube 60 is then inserted a short distanceinto the lower end of the superheat extension tube 102 and welded to theinner surface of the extension tube 102. The reactor superheat tube 60extends downwardly through the active core 46 to a point approximatelyat the center of a lower shield 106 also to be described more fullyhereinafter. The superheat tube 54 is then welded to the lower end ofthe reactor superheat tube 60.

A boiling extension tube 103 is positioned and secured in the samemanner as previously described for the super heat extension tube 102.The reactor boiling tube 64 is also positioned and secured in the samemanner as previously described for the reactor superheat tube 60.However, in the case of the reactor boiling tube 64, a boiling tube 110couples the lower boiling stay 38 to the reactor boiling tube 64 and iswelded at each end to the stay 38 and the tube 64. The boiling tube 110also prevents radiation streaming from the active core 46 to the lowerstayedtube sheet 22.

At the center of each of the reactor superheat tubes 60 and the reactorboiling tube 64 is enclosed at least one fuel assembly 112a and 1121)respectively. A suitable type of fuel assembly 112 and its installationwithin the reactor boiling tube 64 and reactor superheat tube 60 isdescribed and claimed in the copending application of S. N. Tower,Neutronic Reactor and Fuel Element Therefor, filed Apr. 22, 1960, Ser.No. 24,128, now US. Patent No. 3,211,623, issued Oct. 12, 1965, andassigned to the present assignee.

Tubes 54, 60, 64, 102, 108 and 110 together with all fuel elements 112,are supported by both the upper stayedtube sheet 20 and the lowerstayed-tube sheet 22. The reactor 16, in turn, is supported by acircular skirt (not shown) welded to the bottom of the reactor vessel18. The circular skirt, in turn, is supported by concrete foundation(not shown).

A support structure 114 is located at the bottom of the reactor vessel18 and supports the weight of the lower shield 106, a plurality ofelongated graphite sections or cells 116 to be described hereinafter,and the upper shield 104. The support structure 114 comprises an annularplate type support 118, having a right angle crosssection, and acircular plate 120. The annular support 118 is given further rigidity bya plurality of gusset plates 122 welded to the plate support 118. Thecircular plate 120 is positioned at the center of the vertical leg ofthe plate support 118 and welded thereto. The circular plate 120 canalso be supported by a plurality of tubular supports 124 uniformlyspaced under the entire area of the circular plate 120. The tubularsupports 124 are welded to the circular plate 120 and rest on the flatinner plate 26. The circular plate 120 can also be supported by weldingit to the vertical leg of the plate support 118 in combination with theuse of the tubular supports 124. The circular plate 120 also has aplurality of openings to permit the passage of the control rod followers42, the boiling tubes 110, and the external superheat tubes 54. Theannular plate type support 118 has a plurality of openings 125 at theouter periphery of the horizontal leg and also at the bottom of thevertical leg to permit the passage of helium gas from a gas inlet nozzle126 to be described hereinafter to the space between the circular plate120 and the flat inner plate 26. The lower shield 106 is circular inshape and comprises alternate layers of boronated graphite and steel, asshown in FIG. 3 for the upper shield 104. The part of the lower shield106, which is below the horizontal plate of the annular plate typesupport 118, is contained within the vertical leg of the plate support118. The layers of the lower shield 106 above the plate support 118 havean outer diameter which permits the formation of a lower annular passage132 between the outer edge of the lower shield 106 and a sheet metalcover 130 which covers insulation 128. 'Holes are also provided in thelower shield 106 to permit the passage of tubes 54, 60, 64, 110 andcontrol rod drive shaft 42.

Supported by the lower shield 106 are a plurality of the graphite cells116 which have a hexagonal cross-section. The graphite cells 116 have avertical height of approximately one-half the height of the reactorvessel 18. The overall diameter of the entire group of graphite cells116 is the same as the overall diameter at the top of the lower shield106. Each graphite cell 116a has one boiling tube 64, one reactorsuperheat tube 60 or one control rod 44 passing longitudinally throughits center in the region of the active core 46. In this example, theactive core 46 comprises the volume which contains the fuel assemblies112. The graphite cells 116:5 located beyond the outer periphery of theactive core 46 are constructed in the same manner as the graphite cells116a, except for the fact that 'there are no holes longitudinallythrough the center o'f'the cells 116b. The graphite cells 116b (FIG. 3)surrounding the active core 46 and that portion of the graphite cells116a above and below the active core 46 comprise the reflector region ofthe graphite. That portion of the graphite cells 116a within the activecore 46 comprises the moderator region. Supported by the graphite cells1161s the upper shield 104. The upper shield 104 is constructed in thesame manner as previously described for the lower shield 106. The uppershield 104 is also shaped so 'as'to provide an upper'annular passage 134between its outer diameter and the sheet metal cover 130 and also withthe flat inner plate '26 so as to provide a passagefor the helium gas toa gas outlet nozzle 136.

The lower shield 106 and the upper shield 104 are provided to protectthe upper stayed-tube sheet 20 and the lower stayed-tube sheet 22 fromradioactive activation. The reflector region enveloping the active core46 is provided for neutron economy and flux flattening. The moderatorregion within the active core 46 is provided to thermalize fastneutrons.

The inner surface of the reactor vessel 18 is covered with a'blanket ofinsulation 128, which is sufliciently thick to maintain the temperatureof the reactor vessel 18 at approximately the same level as thetemperature of the upper stayed-tube sheet 20 and the lower stayed-tubesheet 22 in order to eliminate the differential expansion problem whichexists between the reactor vessel 18 and the stayed-tube sheets 20 and22. The sheet metal cover 130 is then used to cover the inner surface ofthe insulation 128 to protect the insulation 128 from the flow of heliumgas. Insulation (not shown) can also be provided on the outside upperand lower portions of the reactor vessel 18 8 to decrease thetemperature gradient between the tube sheets 20 and 22 and the reactorvessel 18.

At the lower portion of the reactor vessel 18 is provided the gas inletnozzle 126 to permit the entrance of cooled helium gas. Directlyopposite the gas inlet nozzle 126 is provided the gas outlet nozzle 136to permit the exit of the heated helium gas. The inlet and outlet heliumflows are separated by an annular gas baflle 138, which is horizontallypositioned immediately above the gas nozzles 126 and 136 and is fastenedto the outer periphery of the graphite reflector region. The horizontalgas bathe 138 extends from the outer periphery of the graphite reflectorregion to the inner surface of the sheet metal cover 130. T 0 reduce theleakage between the gas baffle 138 and the sheet metal cover 130 alabyrinth type seal (not shown)- can be used at the sheet metal cover130, A portion of the gas baflle 138 is cut away opposite the gas outletnozzle 136 in order to permit the flow of helium gas from the upperpassage 134 through the gas outlet nozzle 136. A vertical gas battle isthen sealably secured to the outer edge of the horizontal gas bathe 138.The baffle 140 is also sealably secured to the horizontal portion of theplate type support 118, so as to form a passage between the vertical gasbaflle 140 and the gas outlet nozzle 136. The vertical gas baflle 140extends sideways so as to abut against the sheet metal cover 130 toprevent the direct flow of helium from the gas inlet nozzle 126 to thegas outlet nozzle 136 by way of the lower passage 132. For the samereason no holes are provided in the horizontal section of the annularplate type support 118 in the area. enclosed between the vertical gasbaffle 140 and that portion of the reactor vessel 18 containing the gasoutlet nozzle 136.

A flow passageway or annular gas passage 142 (FIG. 3) is providedbetween each tube 60 and 64 and its surrounding shields 104 and-106 andgraphite cells 116 in order to provide a flow path for the helium gasfrom the bottom of the reactor vessel 18 to the top of the reactorvessel 18. The annular gas passage 142 is also provided in the samemanner for each control rod drive shaft 42 and each control rod 44. Theflow of the helium through the annular passages 142, just described,aids in the transfer of heat from the moderator and reflector regions tothe primary coolant flowing through the tubes 60 and 64. This flow ofhelium through the active core also aids in cooling the control rods 44.A plurality of aluminum oxide (A1 0 spacers 144 (FIG. 3) are provided ateach end of the graphite cell 116a. between each reactor boiling tube 64and reactor superheat tube 60 and its corresponding graphite cell 116a.The spacers 144 are threaded into the graphite cell 116a and buttagainst the tubes 60 and 64. Therefore, the tubes 60 and 64 aid inholding the graphite cells 116a in position. The spacers 144 alsoprevent the tubes 60 and 64 from touching the graphite cells 116a andthus producing hot spots and possible burnout in the tubes 60 and 64.The spacers 144 are spaced radially and longitudinally with respect toone another. It is also to be noted that the extension tubes 102 and 108provide shielding against radiation streaming along the gas passages142.

A refueling tank 146 is also provided above the upper stayed-tube sheet20 and is welded to the top of stayedtube sheet 20. A refueling tankbellows joint 148 is provided in the refueling tank 146 in order to takecare of the differential expansion which may occur between the refuelingtank 146 and the upper stayed-tube sheet 20. A reactor vessel bellowsjoint 150 can also be provided in the upper portion of the reactorvessel 18 to take care of differential expansion between the upperstayed-tube sheet 20 and the reactor vessel 18. In order to carry theload when the refueling tank is filled with water, conventional stopscan be provided within the refueling tank bellows joint 148 and thereactor vessel bellows joint 150 to revent the compression of the joints148 and 150 beyond a predetermined point. An alternate method that canbe '9 'use d is to provide separate supports for the upper stayedtubesheet 20 by any conventional method.

Refueling and recycling of the fuel assemblies 112 can be accomplishedin the following manner. First the refueling tank 146 is filled withwater. Then the end plug 50 is removed by cutting a weld 152 (FIG. 3).Remote handling tools are then lowered through the upper boiling stay94, through the boiling extension tube 108 and through the reactorboiling tube 64 down to the top of the fuel assemblies 112k. Lugs,provided at the top of the fuel assembly 112b, can then be grasped bythe remote handling tool (not shown) and the fuel assembly 1121:removed. The fuel assembly 112a in the reactor superheat tube 60 canalso be removed in a similar manner. At this stage the fuel assemblies112 can be relocated from one tube to another or can be replaced by anew fuel assembly 112. The control rod 44 can also be removed in agenerally similar manner except that the refueling tank 146 is dry andthe control rod 44 is Withdrawn into a lead cast (not shown) locatedabove the reactor 16. In addition, the control rod 44 must bedisconnected from the control rod drive mechanism located below thereactor 16.

It is also to be noted that the reactor boiling tube 64 can be removedfrom above the reactor 16 by cutting a weld 154, which holds the reactorboiling tube 64 to the boiling extension tube 108. In addition, the endplug 50 must also be removed from the lower boiling stay 38 so as toprovide access to the weld between the bottom of boiling tube 110 andthe inner surface of the lower boiling stay 38; whereby, the weld can becut and the reactor boiling tube 64 removed by a remote handling toolthrough the top of the reactor 16. The reactor superheat tube 60 can beremoved in the same manner as described for the reactor boiling tube 64,except that the weld between the bellows joint 62 and the superheat tube54 must be cut and the pressure tube 54 must itself be cut to permit theremoval of the reactor superheat tube 60 through the top of the reactor16.

Referring now specifically to FIG. 2, it can be seen that this is a tworegion core; namely, the boiling region and a superheating region. Theboiling region comprises all of the reactor boiling tubes 64 which arelocated within the inner confines of the reference line 86. Thesuperheat region comprises all of the reactor superheat tubes 60 whichare located between the reference line 86 and the reactor vessel 18.

The following tabulation of plant and reactor characteristics and ofmaterials of construction are presented as a guide to the constructionof a reactor plant embodying the present invention with the obviousintent that the tabulation is merely exemplary of an illustrativeapplication of the invention and not limitative thereof. Obviously,differing characteristics and materials can be selected by the nuclearengineer upon the basis of readily available technology, whenconstructing a nuclear plant having a differing power rating.

PLANT CHARACTERISTICS Reactor Heat 47.5 MW.

Gross electric output 16.5 MW. Auxiliary load Q .7 MW.

Net electric output 15.8 MW.

Overall plant elficiency 33.3%.

Turbine heat rate 9740 B.t.u./ kw./ hr. Turbine cycle efliciency 35.0%

Turbine throttle pressure 850 p.s.i.g.

Turbine throttle temperature 900 F.

Reactor heat 1.62 B.t.u./ hr. Steam flow 147,700 lbs./ hr. Feedwatertemperature 380 F.

Boiling recirculation rate 10/1.

Recirculation flow 1.48 X 10 lbs./ hr. Boiling section exit quality,avg. 10%.

Superheated steam line 1-6 in. in. wall.

Reactor vessel design conditions 50 p.s.i.g. 1000 F. Moderator operatingtemperature 11001200 F.

Helium circulating gas flow 15,5 00 lbs./ hr. Helium circulating line1-12 in. Sch. 10. Helium circulating system pressure drop m4 p.s.i.Graphite cylinder height (including reflector) 10 ft. 0 in. Graphitecylinder diameter (including reflector) 9 ft. 4 in. Graphite reflectorthickness -2 ft. 0 in. Active core height 6 ft. 0 in. Active corediameter 5 ft. 7 in. Pressure tube lattice 7 in. A pitch. Number ofpressure tubes 85. Number of boiling tubes 49.

Number of superheating tubes 36.

Pressure tube design conditions 1080 p.s.i.g. at 1000 F.

Pressure tube max. working conditions 900 p.s.i.g. at 900 F. Pressuretube material Type 316 SS. Pressure tube O.D 3.00 in. Pressure tube I.D2.76 in. Pressure tube wall thickness .120 in. Moderator/ U volume ratio23 .9/ 1 Number of fuel rods/ pressure tubes 37.

Fuel rod O.D .350 in. Fuel rod cladding .015 in. SS. Pellet diameter.318 in. Fuel rod spacing (A pitch) .400 in. Weight of U0 6490 lbs.

U enrichment, boiling region 5.5%. U enrichment, superheating region4.5%. Total number of fuel rods 3145. Number of control rods 21. Typecontrol rods 1.70 in. O.D.B C. Core heat transfer surface 1728 ft.Boiling heat transfer surface 996 ft. Superheating heat transfer surface732 ft. Core average heat flux 93,800 B.t.u./ hr. ft.

Boiling region average heat flux 124,500 B.t.u./ hr. ft. Superheatingregion average heat flux 571,700 B.t.u./hr. ft. Core maximum heat flux390,000 B.t.u./ hr. ft. Ratio maximum average heat flux 4.16.

MATERIALS SUMMARY Fuel U02. Fuel cladding Type 316 stainless steel.Pressure tubes Type 316 or 347 stainless steel. Tube sheets Type 304stainless steel. Reactor vessel Type 304 stainless steel. Circulatingwater piping Type 304 stainless steel. Saturated steam piping Type 304stainless steel. Superheater steam piping 2% Cr., 1 Mo. Croloy.

Feedwater piping Al06 carbon steel.

11 MATERIALS SUMMARY-Continued Steam drum A-2l2 with SS clad. ModeratorGraphite-reactor grade. Cover gas Helium. Control rod poison Boroncarbide.

Operation of the stayed-tube sheet type IBSHR Referring now to FIG. 5 ofthe drawings, an operational explanation of IBSHR will be given. To aidin the understanding of this flow circuit a legend has been established,as shown in FIG. 5; wherein, water is indicated by a solid line flowarrow, steam is indicated by a broken line flow arrow, steam-watermixture is indicated by a dot-dash line flow arrow, and helium gas isindicated with a solid line flow arrow with a single wave in the line.Reference can also be made to FIGS. 1, 3 and 4, for a clearerunderstanding of the fiow circuitry within the reactor 16, the upperstayed-tube sheet 20, and between the reactor 16 and the steam drums 76.

A circulating pump 156, located opposite the lower portion of thereactor 16, pumps the primary Water, as indicated by a solid line arrow220, through the circulating pump discharge conduit 34 to the lowerplenum chamber 30. The primary water then flows through the reactorboiling tubes 64 and flows over the fissile fuel assemblies 112b, wherethe primary water is heated by the fuel assemblies 11212. As the primarywater flows through the reactor boiling tubes 64, it also absorbs heatfrom the graphite moderator surrounding the reactor boiling tubes 64.The primary water then changes into a steamwater mixture, as indicatedby a dot-dash line flow arrow 222, which flows into the steam-waterspace 66. From the steam-water space 66 the steam-water mixture flows tothe lower portion of the steam drum 76, which is located opposite theupper portion of the reactor 16. In the steam drum 76, the steam-watermixture is separated into steam, as indicated by a broken line flowarrow 224, and into primary water 220. Since the circulation ratio(pounds of water/ pounds of steam) is approximately ten to one, tenpounds of primary water 220'is returned to the circulating pump 156through a circulating pump suction conduit 158 for every pound ofsubstantially dry, saturated steam 224 flowing to the steam space 68 viathe reactor steam inlet conduit 80. The saturated steam then flowsdownwardly through the reactor superheattubes 60 and absorbs heat fromthe fuel assemblies 112a over which the steam passes and from thegraphite moderator surrounding the reactor superheat tubes 60 to becomesuperheated steam as indicated by a broken line flow arrow 226. Thesuperheated steam 226 then flows from the reactor superheat tubes 60through the superheat tubes 54 into a superheat outlet header 58.

The boiling region of the reactor core 46 does not require orificing,because of the fiat radial flux distribution in the center portion ofthe reactor core 46 and the large margin between the operating-fluxlevel and the burnout heat flux. However, the superheat region of thereactor core 46 does require orificing because of a somewhat steeperflux gradient. Therefore, the temperature of the superheated steam 226leaving the reactor superheat tubes 60 is controlled by an orificemeans, such as a flow control valve 160 located in each of the superheattubes 54. The superheated steam then flows from the superheat outletheader 58, through a superheat conduit .162, through a high pressureturbine trip valve 164, and into a high pressure turbine 166. Thesuperheated steam then flows through the high pressure turbine 166,through a cross-over conduit 16S, and into and through a low pressureturbine 170. The steam while passing through the high pressure turbine166 and the low pressure turbine 170 turns the turbine rotor which thenrotates the generator rotor and produces A.C.'power in the generator172. The low pressure turbine outlet steam then flows 1.2 from the lowpressure turbine into a condenser 174. The steam, as indicated by abroken line flow arrow 229. upon entering the shell side of thecondenser 174 is condensed by cooling water such as river water flowingthrough the condenser tubes. The condensate, as indicated by a solidline flow arrow 238, then flows to a condensate pump 176. The condensatepump 176 then pumps the condensate 238 through a filter 178, through atleast one demineralizer 18! and then to a deaerator 182. The

filter 178 removes corrosion products, the demineralizer 180 removesionized particles, and the deaerator 182 removes noncondensable gaseswithin the condensate 238. The condensate then flows from the deaerator182 by gravity to a boiler feed pump 184. The boiler feed pump 184 thenpumps the condensate as high pressure boiler feed water, indicated bythe solid line flow arrow 240, through a high pressure heater 186, ahelium cooler 188, and back to the steam drum 76. Additional lowpressure and high pressure heaters can be installed respectively aheadof and after the boiler feed pump 184 to increase the thermal efficiencyof the plant. All the heaters receive extraction steam from the turbineto heat the condensate flowing through the low pressure heaters and alsoto heat the boiler feed water flowing through the high pressure heaters.In addition, the helium cooler 188 is used to cool helium and at thesame time to heat the boiler feed water.

An emergency cooling system is also provided to maintain adequatecooling of the active core 46 in the event of loss of power to thecirculating pump 156. In the case of such a failure, water flow ismaintained in the reactor boiling tubes 64 by natural circulation fromthe steam drum 76. The reactor superheat tubes 60 in turn are cooled bythe steam generated in the reactor boiling tubes 64. An emergencycooling valve 190 is then opened to cool the steam produced by thereactor 16. The steam then flows, as indicated by broken line flow arrow242, from the superheat conduit 162, through the emergency cooling valve190, and through an emergency cooling inlet conduit 192 to an emergencycooler 194. The emergency cooler 194 is a submerged coil located in therefueling tank 146, which is located above the reactor 16 and .is filledwith water. The steam is condensed in the emergency cooler 194. Thecondensed steam or water flows, as indicated by solid line flow arrow243 from the'emergency cooler 194, through an emergency cooling outletline 196, through an emergency cooling check valve 198, and returns .to.the steam drum 76. The emergency cooling check valve 198 permits theflow of water 243 only in a direction from the emergency cooler 194 tothe steam drum 76 but will not permit a reverse flow to occur. Duringnormal operation, the emergency cooling valve 190 is closed to preventsteam from bypassing the high pressure turbine 166, and the emergencycooling check valve 198 prevents any water from flowing from the steamdrum 76 into the emergency cooler .194. The

emergency cooler 194 is located in the refueling tank 146 at anelevation sufficiently above the steam drum 76 to permit gravity waterreturn from the emergency cooler 194 to the steam drum 76.

In the event of a loss of power accident (e.g. a turbine trip-out), asteamdump system is used to maintain proper circulation within thereactor 16 with a minimum loss of steam to the atmosphere through thesafety valves which would normally open on a turbine trip-out. Upon aturbine trip-out, the high pressure turbine trip valve 164 closes. Asteam dump valve 200, which is electrically interlocked with the highpressure turbine trip valve 164, opens simultaneously with the closingof the turbine trip valve 164. Steam, as indicated by broken line flowarrow 244, then flows from the superheat conduit 162, through the steamdump valve 200, through a steam dump line 202, through a steam dump 204,and then into the condenser 174. The steam is then condensed in thecondenser 174 as previously described, and the condensate 13 thenfollows the fluid path previously described from the condenser 174 tothe steam drum 76.

In this example, the choice of graphite as a moderator requires amoderator cooling system to: (1) prevent oxidation of the graphite athigh temperatures, (2) limit control rod temperatures, (3) maintain peakgraphite temperatures below the temperature at which a carbonwaterreaction could be significant in the event of a reactor tube rupture,and (4) limit the total thermal capacity of the graphite mass. Toaccomplish this an inert gas (helium) blankets the graphite and iscirculated through gas passages around the reactor tubes, control rods,and the graphite matrix as a heat transfer, heat transport medium. Theheated helium leaves the reactor 16, enters a gas outlet conduit 206,and flows to a blower 208, as indicated by a single wave solid line flowarrow 245. The blower 298 discharges the helium into a blower outletconduit are, which conducts the helium from the blower 208 to the heliumcooler 188. The helium 24.5 enters the shell side of the helium cooler138 and is cooled by passing over tubes within the helium cooler 188through which the boiler feed water flows. After the helium has beencooled in the helium cooler 188, the helium 245 flows through a gasinlet conduit 212 to the reactor 16. The helium enters the reactor 16and is directed downwardly, as indicated by a single wave solid lineflow arrow 246, by the horizontal gas baflie 138. The helium 246 thenmakes a sin le upward pass inside the reactor 16 through the annulargaps between the graphite and the reactor tubes and also between thegraphite and the control rods 44. The helium exits from the active core46 near the top of the reactor 16, as indicated by a single wave solidline flow arrow 247, and then fiows downwardly within the periphery ofthe reactor 16. The helium then flows from the reactor 16 into the gasoutlet conduit 2% to complete the flow cycle of the helium. The verticalgas baifle 14d prevents the helium from bypassing the active core 46,when it first enters the reactor 16. A bypass helium conduit 214, whichcouples the blower outlet conduit 2119 to the gas inlet conduit 212,permits hot helium gas to bypass the helium cooler 188, as indicated bya single wave solid line fiow arrow 248. This permits temperaturecontrol of the helium 246, which enters the reactor 16, by a heliumtemperature control valve 216, which is installed in the bypass heliumconduit 214 and controls the amount of helium that bypasses the heliumcooler 188.

Reheaf-IBSHR Referring now to FIGS. 6 through 9 of the drawings and inparticular to FIG. 6, another modification of the reactor of theinvention is depicted therein. The overall formation of a Reheat-IBSHR259 of FIG. 6 is generally similar to FIG. 1 and consequently similarreference characters have been employed to identify corresponding parts.A reactor vessel 252 is a vertical cylindrical tank equipped with anexternal upper biological shield 254 and a lower biological shield 256.The reactor vessel 252 i is immersed in and cooled by the neutron shieldtank water (not shown) which surrounds it. The upper and lower shields254- and 256, respectively, comprise a combination of water and steelshot 257. The steel shot 257 in the upper shield is contained by acylindrically shaped shell 258, which serves as a reactor vesselextension and has its lower end welded to the reactor vessel 252. Acircular cover plate 260, which rests on the reactor vessel extension258, serves as a cover plate to prevent dirt from getting into the uppershield 25 3. A circularly shaped reactor cover plate 264 rests on aconcrete foundation 262. The reactor cover plate 264 forms the bottom ofa refueling tank (not shown), which is filled with water. A gasket (notshown) can be used as a seal between the lip of the reactor cover plate264 and the concrete foundation 262 to prevent water from leaking by thereactor cover plate 264. The cover plate 264 is also located at a pointsufliciently high above the Reheat-IBSHR 250 to 14 permit a plurality oftubes to pass between the cover plate 264 and the upper shield coverplate 260.

A plurality of vertically positioned boiling tube extensions 26d,superheat tube extensions 270, and reheat tube extensions 2'72 extendfrom a point a short distance below the insulation 128, throughvertically aligned holes in the top of the reactor vessel 252, the uppershield cover plate 263, and the reactor cover plate 264, and extend to apoint a short distance above the reactor cover plate 264. Upon controlrod sleeves 266 are installed in the same manner as the tube extensions268, 270 and 272, except that the control rod sleeve penetrates thereactor 254 to the top of the reflector region.

Referring now in particular to FIG. 7, there is shown a sectionalelevation of the aforementioned boiling tube extension 268 welded to thetop of the reactor vessel 252 and passing slidably through the uppershield cover plate 263 and the reactor cover plate 264. Also shown inFIG. 7 is a bellows joint 62 having one end welded to the reactor coverplate 264 and the other end welded to the extension 268. The bellowsjoint 62 provides for the differential expansion which occurs betweenthe aforementioned extension and the external items between the top ofthe reactor 252 and the reactor cover plate 264. The aforementioneddescription and explanation also applies to the tube extensions 270 and272 and to the upper control rod sleeve 266.

Referring again to FIG. 6, insulation 128 is provided to the entireinner surface of the reactor vessel 252 in the same manner as previouslydescribed for FIG. 1, in order to reduce heat losses to the neutronshield tank water (not shown).

A lower vessel skirt 276, comprising a cylindrically shaped shell andhaving an annular ring welded to the lower end of the shell, is weldedto the lower portion of the reactor vessel 252 and rests on a concretefoundation 262. A cylindrically shaped shell 280 for the lower shield isvertically positioned and has its upper end welded to the inner surfaceof the annular ring which forms part of the lower vessel skirt 276. Acircularly shaped lower shield plate 232 is then positioned horizontallyand welded to the lower end of the lower shield shell 280. The weight ofthe reactor vessel 252, all components within the reactor vessel 252,the upper shield 254, and the lower shield 256, is transmitted to theconcrete foundation 262 through the lower vessel skirt 276. The gasinlet nozzle 126 and the gas outlet nozzle 136 are formed in the reactorvessel 252 in the same manner as previously described for FIG. 1, withthe exception that in this example the gas inlet nozzle 126 has beenrelocated so as to enter the bottom of the reactor vessel 252.

The graphite portion contained within the reactor vessel 252 isgenerally similar to that described for FIG. 1 and comprises a moderatorand reflector region as previously described for FIG. 1. The combinedmoderator and reflector lattice or unit again comprises a plurality ofgraphite cells 234 (FlG. 9). In this example, however, the graphitecells 234 have a square cross-section instead of a hexagonalcross-section as previously described for FIG. 2. Each graphite cell 284Within the active core 46 is provided with a flow passageway or boredpassage 292 to receive a reactor boiling tube 314, a reactor reentrantsuperheat tube 288, or a reactor reheat tube 318. As shown in FIG. 9, acontrol rod passage 2% is formed by a circular hole at the junction offour graphite cells. Also shown in FIG. 9 is a plurality of bored holes296 in the periphery of the graphite cells 284 to permit additional flowof helium over the exterior of the graphite cells 234. Graphite cells inthe reflector region (not shown) do not have any tube passages 292 orgas passage holes 224; however, holes can be provided if cooling of thegraphite cells is necessary in the reflector region. FIG. 9 also showsone control rod passage 294 for every four tube passages 292. Themoderator and retlector lattice may also be a single section of graphitehaving a plurality of flow passageways 292, control rod passages 294,and/ or holes 2% extending therethrough.

The graphite cells 284 are supported by a graphite support structure 274which in turn comprises a plurality of segmented plates to be describedhereinafter. Each segmented plate in turn is supported by an individualtubular support also to be described hereinafter. A tubular sleeve 380is vertically positioned and penetrates the bottom of the reactor vessel252 and the lower shield plate 282 and is welded at both penetrations,as shown more clearly in FIG. 7. A lower control rod sleeve 382 issimilarly positioned and installed as described for the tubular sleeve3%. The lower control rod sleeve 302 is used only when required toreceive the control rods 44 which are inserted into the active core 46from the bottom of the Reheat- IBSHR 250. In this example of the controlrods 44 are inserted upwardly into the active core 46, and 70% of thecontrol rods 44 are inserted downwardly into the active core 46. Eachsleeve 300 and each lower control rod sleeve 302 has a plurality ofelongated holes 304, which permits the flow of helium from a lowerplenum chamber 306 into the. inside of the sleeves 388 and 302. Thelower plenum chamber 306 comprises the space between the bottom of ofthe reactor vessel 252 and the graphite support structure 274. Anannular baffle 310 is fastened to the periphery of the support structure274 and butts against the inside of the insulation 128 in order toprevent the helium which enters the reactor vessel 252 from bypassingthe active core 46.

The control rods 44 are generally similar to those described for FIG. 1.However, in this example, the control rods 44 are driven by hermeticallysealed drum and cable type mechanisms 312, which are all located in therefueling pool above the reactor cover plate 264. The control rods 44inserted upwardly into the active core 46 provide axial flux shapingcapability. However, the control rods 44 inserted downwardly into theactive core 46 provide the gravity scram requirement. The control rods44 can be removed through the top of the reactor vessel 250 by cuttingthe weld between the drum and cable type mechanism 312 and the uppercontrol rod sleeve 266. The drum and cable type mechanism 312 and thecontrol rod 44 can then be removed.

Referring now to FIG. 7, a detailed construction of a reactor boilingtube 314 is shown. The reactor boiling tube 314 is positioned verticallyand extends upwardly in a coaxial manner from a point a short distancebelow the lower shield plate 282 through the tubular sleeve 300, throughthe graphite cell 284, and extends into the boiling tube extension 268to a point a short distance above the upper shield plate 260 at whichpoint the reactor boiling tube 314 is welded to the inner surface of theboiling tube extension 268. One end of the bellows joint 62 is thenwelded to the lower portion of the tubular sleeve .300, and the otherend of the bellows joint 62 is Welded to the reactor boiling tube 314.The fuel assembly 112b is located within the reactor boiling tube 314 ata location midway between the top and bottom of the Reheat- IBSHR 251).Surrounding the reactor boiling tube 314 is the graphite cell 284. Thegraphite cell 284 is supported "by a squarely shaped collar 298, whichis welded to the tubular sleeve 300. The tubular sleeve 3% passesthrough and extends a short distance beyond the collar 298. The sleeve300 fits into an inner annular offset formed at the lower end of thegraphite cell 284 and acts as a lateral guide for the cell 284. Thecollars 298 form part of the previously mentioned graphite supportstructure 274. The collar 298 can also be made so that a plurality ofgraphite cells 284 are supported by only one collar 298 which has aplurality of tubular sleeves 300 welded thereto.

As indicated in FIG. 7, the annular gap surrounding the reactor boilingtube 314 permits the flow of helium upwardly from the tubular sleeve 300over the surface of the reactor boiling tube 314, as indicated by flowarrow 249. Also shown is the flow of primary water 220, which enters thebottom of the reactor boiling tube 314 and flows upwardly over the fuelassembly 11217. The water then absorbs heat from the fuel assembly 112band is transformed into a steam-water mixture 222. The steamwatermixture continues to flow in an upwardly direction, leaves the reactorboiling tube 314, and enters the boiling tube extension 268. Thesteam-water mixture 222 then leaves the boiling tube extension 268through a boiling tube extension outlet nozzle 316. l

Returning now to FIG. 6, a reactor reheat tube 318 is constructed in thesame manner as the reactor boiling tube 314 described in FIG. 7.However, in operation, slightly superheated steam enters the reactorreheat tube 318 through a reheat tube extension inlet nozzle 320. The

'steam'then flows downwardly inside the reactor reheat" tube 318 andpasses over the fuel assembly 1120, contained within the reactor reheattube 313, where the steam absorbs heat from the fuel assembly. Thesteam, upon the absorption of heat, becomes further superheated steamand continues to flow downwardly through the reactor reheat tube 318.The refueling or recycling of fuel assemblies 112, contained in thereactor boiling tubes 314 and the reactor reheat tubes 318, isaccomplished in the same manner as previously described for the reactorsuperheat tubes 60 in FIG. 1. The removal of a reactor boiling tube 314or the reactor reheat tube 318 is also accomplished in the same manneras previously described for the reactor superheat tubes 60 in FIG. 1.

Referring now to FIG. 8, there is shown a typical arrangement for areactor re-entrant superheat tube 288, which is exemplary of theplurality of reactor reentrant superheat tubes 288 contained within thesuperheat region of the reactor 250. A tubular support 324 extends fromthe torispherical bottom head of the reactor vessel 252 vertically to asquarely shaped plate 322, which is located adjacent to and in the samehorizontal plane as the previously described collars 298. The plate 322also forms part of the previously mentioned graphite support structure274. The tubular support 324 is welded to the bottom head of the reactorvessel 252 and is also welded to the square plate 322. The tubularsupport 324 also has a plurality of elongated openings 304 which permitpassage of helium from the lower plenum chamber 306 to the inside of thetubular support 324. An annular ring 326, which is vertically above anddirectly in line with the tubular support 324 is placed on top of thesquare plate 322 and welded thereto. The graphite cell 284, which has anoffset annular space at its lower end formed so as to fit over theannular ring 326, is then vertically positioned on and supported by thesquare plate 322. The annular ring 326 serves as a guide to hold thegraphite cell 284 in a fixed lateral position.

'Positioned coaxially within the graphite cell 284 is the reactorre-entrant superheat tube 288, which forms an annular gap between there-entrant superheat-tube 288 and the graphite cell 284 to permit theflow of helium therein. The re-entrant superheat tube 288 comprises anouter re-entrant tube 328 and an inner reentrant tube 330, the innerreentrant tube 330 being vertically disposed and coaxially displacedfrom the outer reentrant tube 328 so as to form an annular passagebetween the inner and outer tubes 328 and 330, respectively. The annularpassage between the inner tube 330 and the outer tube 328 serves as aflow path for saturated steam from a superheat tube inlet nozzle 277 tothe lower portion of the outer reentrant tube 328, as indicated by flowarrow 224.

The outer reentrant tube 328 extends from a point a short distance belowthe active core 46 to a point a short distance above the upper shieldcover plate 260 and within the superheat tube extension 270. The upperend of the outer reentrant tube 328 is open, but the lower end of thetube 328 is sealed by a hemispherically shaped cap 338 welded thereon.To the bottom of the cap 328 is welded a cylindrically shaped reentranttube guide pin 332, which extends downwardly through a hole 334 formedin the center of the square plate 322. The reentrant tube guide pin 332extends downwardly only a short distance below the square plate 322 andserves as a guide to maintain the lateral positions of the outerreentrant tube 328. The square plate 322 also contains a plurality ofholes 336 within it which permit the flow of helium from inside thetubular support 324 to the annular gap formed between the outerreentrant tube 328 and the graphite cell 284.

Returning now to the inner reentrant tube 330, the inner tube 330extends upwardly from a point at the bottom of the active core 46,through the outer reentrant tube 328, and coaxially into the superheattube extension 270 to a point between the superheat tube inlet nozzle277 and a superheat tube outlet nozzle 278, where the inner reentranttube 330 is welded to the inner surface of the superheat tube extension270. The portion of the inner tube 330, which extends above the outerreentrant tube 328, forms an annular passage between the inner reentranttube 330 and the superheat tube extension 270 so as to provide the upperportion of the flow path from the superheat tube inlet nozzle 277 to thelower portion of the outer reentrant tube 328. The inner reentrant tube330 has an open end at the top and a lip 340 at the bottom which extendsradially inwardly to provide a support for the fuel assembly 112a and topermit the flow of saturated steam from the lower portion of the outertube 328 into the inner tube 330. A fuel assembly 112a is locatedvertically in the same relative position as previously described for thefuel assembly 112!) in the reactor boiling tube 314; The steam, whichenters the inner reentrant tube 330, flows over the fuel elements of thefuel assembly 112a, absorbs heat, and becomes superheated. The superheatsteam then flows in an upwardly direction out of the top of the innerreentrant tube 330 and into the upper portion of the superheat tubeextension 270 from which point the steam exits through the superheattube outlet nozzle 278.

Returning now to FIG. 6 of the drawings, a neutron shield tank (notshown), containing water for the absorption of neutrons that escape fromthe reactor 250, surrounds the outer vertical periphery of the uppershield 254 and the reactor 250. Vertically surrounding the neutronshield tank (not shown) is a concrete biological shield (not shown).Outside the biological shield and above the reactor 250 are located thesteam drum 76, the superheat outlet header 58 and a reheat outlet header344. Outside the biological concrete shield (not shown) and below thereactor 250 are located the circulating pump 156, a circulating waterheader 352, and a reheat inlet header 342. The circulating pump suctionconduit 158 couples the bottom of the steam drum 76 to the circulatingpump 156. The circulating pump discharge conduit 34 then couples thecirculating pump 156 to the circulating Water header 352. A boilinginlet tube 286 for each of the reactor boiling tubes 314 individuallycouples each of the reactor boiling tubes 314 to the circulating waterheader 352. Two adjacent boiling tube extension outlet nozzles 316 arecoupled to a single Y connection 354, which in turn is coupled to thesteam drum 76 by a single boiling outlet tube 346. Two boiling tubeextension outlet nozzles 316 are coupled to a single boiling outlet tube346 in order to reduce the number .of boiling outlet tubes 346 requiredto couple the outlet flows from the reactor boiling tubes 314 to thesteam drum 76.

Each of the reactor reentrant superheat tubes 288 is coupled to thesteam drum 76 by a superheat inlet tube 348 in order to permit the flowof saturated steam from the steam drum 76 to the superheat tubeextension 270. In each of the superheat inlet tubes 348 is installed anorificing means such :as a superheat inlet tube valve 358, which is usedto control the flow of steam through the reactor reentrant superheattubes 288 in order to control the outlet temperature of the superheatsteam as it flows from the reactor reentrant superheat tube 288. Each ofthe superheat tubes outlet nozzles 278 are then coupled to the superheatoutlet header 58 by a superheat outlet tube 350 in order to permit theflow of superheated steam from the reactor reentrant superheat tube 288to the superheat outlet header 58.

The lower end of each of the reactor reheat tubes 318 is coupled to thereheat inlet header 342 by the reheat inlet tube 290 in order to permitthe flow of reheat steam from the reheat inlet header 342 to the reactorreheat tubes 318. In each of the reheat inlet tubes 290 is installed anorificing means such as a reheat inlet tube valve 360, which is used tocontrol the flow of reheat steam through the reactor reheat tube 318 asa means of controlling the outlet temperature of the reheat steamleaving the reactor reheat tube 318.

Each of the reheat tube extension outlet nozzles 320 are coupled to thereheat outlet header 344 by a reheat outlet tube 356 in order to permitthe flow of reheat steam to the reheat outlet header 344 from thereactor reheat tube 318.

The following tabulation of plant and reactor characteristics and ofmaterials of construction are presented as a guide to the constructionembodying the present invention of a Reheat-IBSHR with the obviousintent that the tabulation is merely exemplary of an illustrativeapplication of the invention and not limitative thereof. Obviously,differing characteristics and materials can be selected by the nuclearengineer upon the basis of readily available technology, whenconstructing a nuclear plant having a differing power rating.

REACTOR DATA SUMMARY [340 MWE Reheat IB SHR] Description Units Boilersuperheat Reheater Heat Balance:

Total Reactor Power MWT B20 Reactor Power MWT 397 303.5 119. 5 GrossTurbine Power MWE. 355 Net Plant Power. MWE 334 Net Plant Efi Percent40.7. Turbine Cycle Conditions:

Throttle Temperature F 1,000 Throttle Pressure 0 Total Steam Flown.-.Condenser Back Press in. Hg abs 1 Final Feedwater Temp F 506 NumberFeedwater Heating Stages" N0 7 Reheat Temperature. F 1,000 ReheatPressure (Turbiueiniet) psi 5. -675 Reactor Description- Reactor Vessel:

Inside Diameter it 26.6 Inside Height-.. tt 323.. Wall Thickness in1.125 Material. 304 SS Design Pressure p s i E 100.. Design TemperatureF 230 REACTOR DATA SUMMARY-Continued Description Units B oiler SuperheatReheater Reactor DescriptionContinued Active Core:

Active Equivalent Diameter Final U-235 Enrichment.-. Moderator to FuelVolume R Moderator Coolant. Water and Steam Steam Steam. Reflector:

MateriaL..- Graphi Axial Thickness it.. -2 Radial Thiclrne s it..- .l--2 Pressure Tubes:

Total Numbers. 244 Number. 104.. 72 68 Material. Zirconium ZirconiumAlloy Niobium Alloy. A110 7 Type Through Re-entrant Through InsideDiameter in 5.10 5.10 4. Wall Thickness. in .490 080 Design Pressure...p.s.i.g 3,000 900 Design Temperature F..... 700 1, 200 Fuel Elements:

Fuel Material. Fuel Element Geometry Clad Material Zirca1oy-4- 304 SSClad Thickne in .025 012 Fuel "Meat Thickness in .160 195 Fuel Clad Gapin 0 0 Fuel Assemblies:

Total Number 208 144 136 Number of Elements (annular rings) 7 4 per'Assy. Diameter of Assembly. in 4.86 3. 8i Lattice Spacing in Core in 1414 End Fitting Materials. Zr 304 SS Reactor Control: i

Absorber Material.

Number of Control Rods 69 Cross Sectional Dimensions in 4' 0D. x 2' LDEflective Length it. 16 Type of Drive. Drum and Cable..-

Performance Data:

Reactor Coolant Outlet Temperature.. F 1,000 Reactor Coolant InletTemperature. F 681 Primary System Operating Pressure... .s.i.a 2,725Primary Coolant Flow lb./hr 2,482,000. Avg. Coolant Velocity-CoreInlet.. ft./sec 39.0 Max. Fuel Center Temp F 3,520 Max CladTemperatu1e.. F... ,240

Max Core Heat Flux B.t.u./hr./it. 402,000 Avg. Core Heat FluxB.t.u./hr./it."... 159,

Avg. Core Power Density. kw./it. 19

Peak to Avg. Power Ratio 2.53.

Avg. Specific Power kwtJkg. U--.. 5

Avg. Fuel Burnup MWD/MI... 17,000...--.- 16,000

PRIMARY SYSTEM COMPONENTS Description: Number Circulating pumps 4 Steamdrums 2 Superheat outlet headers 2 Reheat inlet headers 2 Reheat outletheaders 2 Operation of the Reheat-IBSHR Referring now to FIG. 10 of thedrawings, an operational explanation of the Rehcat-IBSHR 250 will begiven. To aid in the understanding of this flow circuit the legend aspreviously described for FIG. 5 is used again in this instance.Reference can also be made to FIGS. 7 and 8 for a clear understanding ofthe flow circuitry within the reactor boiling tube 314 and the reactorreentrant superhcat tube 288.

The circulating pumps 156 pump the primary Water, as indicated by solidline flow arrow 220, through the circulating pu-mp discharge conduits 34to the circulating water headers 352. The primary Water then flows fromthe circulating water headers 352 through the boiling inlet tubes 286 tothe reactor boiling tubes 314. As the water flows through the reactorboiling tubes 314-, it is converted into a steam-water mixture, asindicated by a dot-dash flow arrow 222, by means previously describedfor FIG. 5. The steam-water mixture then flows from the reactor boilingtubes 314 through the boiling outlet tubes 346 to the lower portions ofthe steam drums 76. In the steam drums 76 the steam-Water mixture 346 isseparated into steam, as indicated by a broken line arrow flow 224, andinto primary water 220. From the steam drums 76, th primary water flowsby gravity to the circulating Pumps 156, and the substantially drysaturated steam 224 flows through the superheat tube inlet valve 358,through the superheat inlet tubes 348, and into the reactor rc-cntrantsuperheat tubes 288. As previously described for FIG. 8, the steam flow224 within the rc-entrant superheat tube 288 is downwardly in an outerannular passage to the lower portion of the reactor reentrant tube 288at which points the steam reverses its flow direction and passesupwardly through the center of the reactor reentrant superheat tube 288.As the steam flows upwardly through the center of the reactor reentrantsuperhcat tubes 288, the saturated steam 224 passes over the fuelassemblies 112a contained within the reentrant tubes 288 and absorbsheat in the same manner as previously described in FIG. 5 to becomesuperheated steam as indicated by the broken line flow arrow 226'. Flowcontrol of the steam flowing through the reactor rcentrant superheattubes 288 with the resulting temperature control of the superheatedsteam 226 is accomplished by the superheat inlet tube valves 358 in thesame manner as previously described for FIG. 5. The superheated steamthen flows from the reactor reentrant superheat tubes 28 8 to andthrough the high pressure turbine 166 in the same manner as previouslydescribed for FIG. 5. The outlet steam from the high pressure turbine166 then flows through a turbine outlet valve 362, through a reheatinlet conduit 364 and into the reheat inlet headers 342, as indicated bya broken line flow arrow 228. The superheated steam then passes from thereheat inlet headers 342, through reheat inlet tube valves 360, throughthe reheat inlet tubes 290, and into the reactor reheat tubes 318. Thesuperheated steam then flows upwardly over the fuel assemblies 112e,contained within the reactor reheat tubes 318, and absorbs heat asdescribed hereinbefore resulting in a rise in the steam temperature toproduce reheat steam. The reheat steam then flows from the reactorreheat tubes 318 through the reheat outlet tubes 356, and into thereheat outlet headers 344, as indicated by the broken line flow arrow230. The reheat steam 230 then flows from the reheat outlet headers 344,through a reheat outlet conduit 368, through a low pressure turbine tripvalve 366, and into the low pressure turbine 170. As previouslydescribed for FIG. 5, the steam which passes through the high pressureturbine 166 and the low pressure turbine 170 gives up its heat energy byturning the turbine rotor, which in turn causes the generator rotor torotate and thus produce electrical energy within the generator 172. Thesteam, after passing through the low pressure turbine 170 is condensedin the condenser 174. The condensate then follows a flow circuitry backto the steam drums 76 as previously described for FIG. 5.

The flow circuitry for the helium gas is generally similar to the flowcircuitry described for FIG. 5.

In case of a turbine trip-out, the high pressure turbine trip valve 164,the turbine outlet valve 362, and the low pressure turbine trip valve366 all close automatically. Simultaneously, the steam dump valve 200,which is electrically interlocked with the turbine valves 164, 362 and366, opens and permits the reheat steam 230 from the reheat outletconduit 368 to flow through the steam dump valve 200, through the steamdump conduit 202, and into the steam dump 204, as indicated by thebroken line flow arrow 232. This is similar to the steam dump Systempreviously described for FIG. however, the Reheat- IBSHR 250 faces adifficult emergency cooling problem because of the necessity ofsupplying steam 228, as steam coolant at the proper temperature, to thereactor heat tubes 318 in the reheat region of the active core 46.

To accomplish this, a turbine bypass arrangement comprising a turbinebypass valve 370, a turbine bypass conduit 372, and a desuperheater 374is provided which couples the superheat conduit 162 to the reheat inletconduit 364 at a point down stream of the turbine outlet valve 362. On aturbine trip-out, the turbine bypass valve 370, which is electricallyinterlocked in a similar manner as the high pressure turbine trip valve164, opens fully and permits superheated steam 226 to How from thesuperheat conduit 162, through the turbine bypass valve 370, through theturbine bypass conduit 372, through the desuperheater 374, and into thereheat inlet conduit 364, as indicated by the broken line flow arrow234. In addition, this turbine bypass arrangement requires a highlyreliable source of injection water for the desuperheater 374. This watercan best be supplied by the boiler feed-pumps 184, which must bereliable for other emergency injection requirements as well. In thisexample, two motor driven boiler feed water pumps, with separateelectrical power supplies, and one steam turbine driven boiler feedpump, are provided to guarantee this service. The water supply for thedesuperheater 374 is provided by an arrangement which comprises adesuperheater flow control valve 376 and a desuperheater inlet conduit378, which Couples a boiler feed conduit 185 at a point between theboiler feed pump 184 and the high pressure heater 186 to thedesuperheater 374. Upon a turbine trip-out the desuperheater flowcontrol valve 376, which is electrically interlocked in the same manneras the turbine bypass valve 370, opens and controls the amount of waterthat flows from the boiler feed pump 184, through the desuperheater flowcontrol valve 376, through the desuperheater inlet conduit 378, and intothe desuperheater 374, as indicated by the solid line flow arrow 236.The desuperheater water 236, which flows into the desuperheater 374, iscontrolled by the desuperheater flow control valve 376 in order toreduce the temperature of the desuperheater steam 234, which enters thedesuperheater 374 so as to provide steam 228 of the proper temperatureto the reactor reheat tubes 318.

The reheat dump steam 232 flows through the steam dump 204 to thecondenser 174 until the reactor power is reduced to the 5% level in thisexample. At this point, the emergency cooling system previouslydescribed for FIG. 5 is capable of absorbing the entire amount of heatproduced by the Reheat-IBSHR 250. The emergency cooling system for theReheat-IBSHR 256 cannot provide a gravity flow to the steam drum 76.Therefore, a drain pump 380 is provided in the emergency cooling outletline 196 to provide the diiferential pressure required to pump thecondensate from the emergency cooler 194 to the steam drum 76. The drainpump 380 is necessary, because the pressure in the reheat outlet conduit368 is much lower than the pressure in the steam drum 76 and asufficient head of water cannot be provided so :as to utilize only agravity feed from the emergency cooler 194 to the steam drums 76.

In this example of the Reheat-IBSHR 250, the active core in plan viewcomprises three regions; a boiling region, a superheat region and areheat region. The boiling and superheat regions are the same aspreviously described for FIG. 2. The reheat region, in this example, isgenerally annular in configuration and surrounds the superheat region.As has been described hereinbefore, primary water first passes throughthe boiling region where the water absorbs heat and is converted into asteam-water mixture. Saturated steam is then separated from thesteam-water mixture in the steam drum 76. The saturated steam then flowsthrough the superheat region, where it absorbs heat and is convertedinto superheated steam. The superheated steam is then passed through thehigh pressure turbine, where the superheated steam gives up a portion ofits heat energy in the production of electrical energy. The superheatedsteam is then passed through the reheat region, where it absorbs heatand the temperature of the superheated steam is raised. The reheat steamis then passed through the low pressure turbine, where the heat energyof the reheat steam is utilized in the production of electrical energy.

While there have been shown and described what are, at present,considered to be the preferred embodiments of the invention,modifications thereto will readily occur to those skilled in the art.For example, it is also to be noted with reference to FIG. 6 that theReheat-IBSHR 250 can be so constructed to produce only superheated steamby the elimination of the reactor reheat tubes 318 so as to eliminatethe reheat region of the Reheat-IBSHR 250. It is also possible to changethe reactor re-entrant superheat tubes 288 to the straight-through tubetype similar to the construction of the reactor boiling tubes 314.

Also, in another modification of this invention the upper stayed-tubesheet 20 can be made large enough to contain the vapor separating meansas an integral part of the tube sheet 20. Therefore, the external vaporseparating means and external conduits connecting the vapor separatingmeans to the upper stayed-tube sheet 20 can be eliminated.

Since numerous changes may be made in the above described apparatus anddifferent embodiments of the invention may be made without departingfrom the spirit thereof, it is intended that all matter contained in theforegoing description or shown in the accompanying drawings, shall beinterpreted as illustrative and not in a limiting sense.

What is claimed as new is:

1. A neutronic reactor comprising a pressure vessel, a lattice offixedly positioned solid moderating material disposed within saidvessel, said moderator lattice forming a plurality of flow passagewaysextending therethrough, a

1. A NEUTRONIC REACTOR COMPRISING A PRESSURE VESSEL, A LATTICE OFFIXEDLY POSITIONED SOLID MODERATING MATERIAL DISPOSED WITHIN SAIDVESSEL, SAID MODERATOR LATTICE FORMING A PLURALITY OF FLOW PASSAGEWAYSEXTENDING THERETHROUGH A PLURALITY OF PRESSURE TUBES EXTENDING THROUGHSAID VESSEL AND POSITIONED RESPECTIVELY IN AT LEAST SOME OF SAIDPASSAGEWAYS AND BEING MOUNTED IN SPACED RELATION WITH THE PERIPHERALWALL OF SAID PASSAGEWAYS TO FORM A FIRST FLOW PATH MEANS THEREBETWEEN,FISSILE MATERIAL DISPOSED WITHIN YET SPACED WITH THE WALLS OF SAIDPRESSURE TUBES WITH THE SPACE BETWEEN SAID FISSILE MATERIAL AND SAIDPRESSURE TUBE WALLS FORMING AT LEAST A PORTION OF A SECOND FLOW PATHMEANS THEREBETWEEN, A PRIMARY COOLANT FLOWING THROUGH SAID PRESSURETUBES ALONG SAID SECOND FLOW PATH MEANS WHEREBY SAID PRIMARY COOLANT ISHEATED BY SAID FISSILE MATERIAL, A SECONDARY FLUID FLOWING EXTERIORLY OFSAID PRESSURE TUBES THROUGH SAID FIRST FLOW PATH MEANS AND IN HEATEXCHANGE REELATIONSHIP WITH SAID MODERATOR LATTICE AND SAID PRIMARYCOOLANT, SAID PRESSURE TUBES BEING GROUPED TO FORM A FIRST GROUP FORBOILING THE PRIMARY COOLANT FLOWING THERETHROUGH AND SECOND GROUPSUPERHEATING THE COOLANT FLOWING THERETHROUGH, ONE OF SAID GROUPS BEINGDISPOSED THROUGH THE CENTRAL REGION OF SAID PRESSURE VESSEL, THE OTHEROF SAID GROUPS BEING ARRANGED PARALLELLY AND OUTWARDLY OF SAID ONE GROUPSO THAT SAID PRIMARY COOLANT ENTERS SAID FIRST GROUP OF PRESSURE TUBESIN A LIQUID FORM AND IS HEATED BY THE FISSILE MATERIAL IN SAID FIRSTGROUP OF PRESSURE TUBES TO PRODUCE A VAPOR, MEANS FOR CONDUCTING SAIDVAPOR FROM SAID FIRST GROUP OF TUBES TO SAID SECOND GROUP OF TUBES TOTHAT THE VAPOR IS SUPERHEATED BY THE FISSILE MATERIAL IN SAID SECONDTUBE GROUP AND MEANS FOR REMOVING SAID SUPERHEATED VAPOR FROM SAIDSECOND TUBE GROUP.